This invention relates generally to the field of electrical connectors, and particularly to an apparatus for establishing a mechanical and electrical connection between an electrical connector and a substrate, such as a Printed Circuit Board (PCB), and a method of attaching a column of solder to a connector terminal to form a solder column.
The electrical terminals of a connector may be mechanically and electrically connected to electrical contact pads on a PCB by several conventional methods, including a pin grid array (PGA), land grid array (LGA), and ball grid array (BGA). In addition, the circuit industry connects packages to a substrate by any of the above methods, as well as column grid array (CGA). The present invention is directed at improving the reliability of the mechanical/electrical connection formed between an electrical connector and a substrate, such as a PCB.
In the electrical connector industry, a Ball Grid Array (BGA) type of electrical connector typically has a plurality of solder balls that interconnect the connector to a PCB. A typical BGA connector consists of a substrate having an array of electrical contact pads or traces disposed about the substrate""s surface. Using existing techniques, spherical shaped solder balls are attached to the terminals of the connector by first applying a resin flux to the electrical contact pads, positioning the solder balls onto the electrical contact pads, and running the connector through a reflow furnace. During the reflow process the solder balls are held in position by the flux and wetted onto the electrical contact pads. In addition to holding the solder balls in position, the flux promotes the wetting of the solder balls to the contact pads and chemically cleans the contact pad surfaces.
After having passed through the reflow furnace, the solder portion located near the solder ball and electrical contact pad interface flows to form an electrical and mechanical connection, but in all other respects, the solder balls generally maintain their spherical shape. In lieu of using resin flux, other methods are known that utilize supporting fixtures to hold the solder balls in position during reflow with or without the use of flux.
One problem associated with using solder balls is that, because of their spherical shape, they tend to form short connections that have generally short lengths which may break under stress. As a result, the electrical connection may be interrupted between the connector and the PCB.
In the integrated circuits and in the packaging industries, other methods are also used to form the electrical and mechanical connection between an electronic package and a substrate. One such method currently used for ceramic microprocessor packaging includes the use of Ceramic Column Grid Array (CCGA). The CCGA adds additional height (e.g., physical distance) to the connection that results in additional compliance. In particular, CCGA is most often used in the microelectronics industry in applications where large packages are involved.
A problem associated with the use of CCGA for connection of an electrical connector to a PCB is the difference in how each component reacts to temperature changes. The coefficient of thermal expansion (CTE) is a number that represents the dimensional change of a material per degree of temperature change. By way of example, in a typical connection between an electrical connector and a PCB, a different CTE will exist as between the connector material, the material of the solder joint, and the PCB material. Area array components (connectors/devices/packages, etc.) are typically limited in size based on the effects of differential CTE of the associated materials. This relates to the performance and reliability of the electrical connections. The greater the differential displacements created by CTE mismatch during thermal changes the greater concern for the electrical integrity of the system.
Typically, a well accepted mounting arrangement for mounting a surface mount electrical connector having terminals depending therefrom is to solder each of the metallic terminals onto a copper clad contact pad on the PCB material using a lead and tin alloy solder paste. The thermal profile during solder paste reflow typically exceeds 200xc2x0 C. for short periods of time. The CTE of the epoxy and glass PCB material is different than the CTE of the metallic material of the terminal and is usually also different than the CTE of the solder alloy that joins the two. Effects of this thermal mismatch are first encountered during the cool down period after the solder is reflowed during initial attachment. The molten solder will solidify just below 200xc2x0 C., and at that time the assembly is mechanically fixed and as near stress free as possible. As the temperature continues to decrease, the differing expansion rates begin to become a factor. The PCB material, the solder alloy, and the electrical connector all react at different rates to this change in temperature. As the relative locational association of the solder pads between the terminal and the PCB begins to change, mechanical stress increases in all three components (the connector, the PCB, and the solder joint).
CCGAs include a plurality of solid solder columns that are positioned between the electrical contact pads of a package and a PCB. The connection between the electrical contact pads on a package and PCB are generally formed by first applying a resin flux to the electrical contact pads, positioning the solder columns over the pads, and running the unit through a reflow furnace. The solder column height, which is greater than the diameter of a typical solder ball, allows for greater flexibility in establishing the connection between a package and a PCB. By allowing for greater flexibility, the solder columns reduce stresses at the solder column/electrical contact pad interfaces which, in turn, results in higher product reliability.
Increasing the height of a column with a fixed footprint spreads the total stress over a greater mass and ultimately decreases the stress per unit mass. The column addresses the relative movement between the connector and the PCB by bending as opposed to shearing.
In addition, a problem exists as to how to maintain the proper coplanarity between the mounting surfaces of the connector and substrate, especially as the size of the electrical connector increases. Conventional methods do not provide a method of establishing a coplanar soldering surface regardless of substrate flatness.
What is still needed is an apparatus and method which solve the aforementioned problems associated with forming reliable connections between an electrical connector and a PCB. As will be seen, the present invention provides an improved apparatus and method for forming column grid arrays which solve the aforementioned problems.
The present invention is directed to an electrical connector having a column grid array for connecting an electrical connector to a substrate, such as a PCB. This invention provides the benefits of a solder column to provide a compliant connection between an electrical connector and a substrate, thereby increasing the reliability of the connection, particularly for increasing connector sizes. The solder column is formed such that a portion of the terminal extends into the column of solder. This provides mechanical rigidity to the solder column while at the same time also providing a compliant connection between the connector and the PCB.
The electrical connector apparatus for establishing a connection between a terminal and a substrate has a column of solder that is attached to at least a portion of the distal end of the tail of an electrical contact terminal of the connector. The column of solder is attached to the tail to form a solder column which is adapted for making an electrical and mechanical connection with an electrical contact pad or trace on the substrate. Increasing the height of the solder column improves compliance in the connection between the terminal and the substrate. This feature of improved compliance allows the terminal to absorb higher stresses present between the substrate and the connector body thereby avoiding fracture of the solder joint between the terminal and the substrate. In addition, the terminal tail acts to reinforce the solder column and this metal core of the solder column increases the mechanical properties of the attachment between the connector housing and the substrate.